a. Field of the Invention
The invention relates to laser recording media, and more particularly to a broadband reflective silver data recording and storage medium.
b. Prior Art
Previously, many types of optical recording media have been developed for laser writing. For example, an article in Optical Engineering, Vol. 15, No. 2, March-April 1976, p. 99 discusses properties of a large number of media. Some of these media require post write processing before they can be read, and some can be read immediately after laser writing. The media of interest herein are for "direct read after write" capability, commonly known as "DRAW" media. Presently known laser DRAW media are thin metal films in which holes may be melted, composite shiny films whose reflectivity at a spot may be reduced by evaporation, thin films of dyes or other coatings which can be ablated at a spot, and dielectric materials whose refractive-index may be changed at a point, causing a scattering of light when scanned with a read laser.
The most common DRAW media are thin metal films, usually on a glass substrate. Thin metal films have several advantages: First, they can be produced for research purposes in small quantities with commercially available sputtering equipment. Second, they can be read either by reflection or by transmission. Third, films of tellurium and bismuth have relatively high recording sensitivies.
Attempts have been made to improve the laser recording sensitivity of various types of reflective electrically conducting metal layers. In U.S. Pat. No. 3,911,444 Lou, Watson and Willens disclose a vacuum-deposited metal film recording media for laser writing incorporating a separately deposited plastic film undercoat between the metal film and a flexible transparent substrate to thermally insulate the metal layer in order to require less energy to write with a laser.
In U.S. Pat. No. 3,314,073 Becker discloses a data recording and storage system which consists of laser recording on a dark vaporizable film. This film is to be made as absorptive as possible with as little reflectivity as possible. In U.S. Pat. No. 3,893,129 Endo discloses a laser recording system which records on exposed and developed microfilm. In U.S. Pat. No. 3,689,894 Laura and Eng disclose an image storage and retrieval system. This system records data on developed microfilm by optically writing transparent bits in black areas of the microfilm. This is accomplished by burning holes through the black developed microfilm.
N. V. Philips of The Netherlands and Hitachi of Japan have announced a pre-recording or pre-grooving method using tellurium. Before sputtering the tellurium coating the plastic substrate is molded with 1/8 wavelength depressions at the wavelength of the laser to be used to create regions of altered phase shift of the reflected light, thereby creating tracks that can be followed with an optical servo.
These thin metal films have enabled a large amount of research to be conducted and progress to be made in the design of optical data storage systems. To date, tellurium and amorphous mixtures thereof have evolved as the most widely used of the metal films. However, tellurium must be manufactured by an expensive, batch-type, vacuum sputtering technique; it does not form a tenacious coating; and it introduces manufacturing and environmental complications because of its toxicity and since it rapidly oxidizes in air it must be encapsulated in an airtight system in order for it to achieve an acceptable archival life.
What is particularly desirable about tellurium is that it has a low melting temperature for a metal, 450.degree. C., and also a very low thermal conductivity of 2.4 watts per meter per degree Kelvin at 573.degree. K. For example, in comparison, silver metal has a melting temperature of 960.degree. C. and a thermal conductivity of 407 watts per meter per degree Kelvin at the same elevated temperature. When thin, electrically conductive films of these two metals are compared for laser recording with short pulses of laser power, the tellurium is far superior from a recording sensitivity standpoint since the low thermal conductivity keeps the heat generated by the laser beam confined to a small area and the lower melting temperature facilitates the melting of the hole.
The use of an electrically conducting, continuous metal film of silver as a reflective laser recording medium would be impractical for precisely the same reasons that tellurium has been adopted. That is, silver melts at more than twice the temperature and has a thermal conductivity about 170 times higher. Despite these apparent disadvantages, non-filamentary silver can be used quite effectively if it is dispersed throughout a dielectric such as gelatin with a sufficiently high volume concentration to create a reflective surface but low enough in volume concentration that the silver layer is not continuous. Under these special circumstances the laser beam need only melt the dielectric to record data on the reflective surface, not the silver itself.
A reflective silver laser recording medium of this general type was the subject of a prior application Ser. No. 012,235 by J. Drexler. In that application a processed black filamentary silver emulsion was converted to a reflective non-electrically conductive reflective recording medium by heating at a temperature in the range of 250.degree. C. to 330.degree. C. in an oxygen containing atmosphere until the surface developed a shiny reflective appearance. This laser recording material worked effectively with laser beams of visible wavelength but its recording sensitivity fell by about a factor of three for semiconductor lasers, which generate light beams in the near infrared at about 830 nm. Also the high temperature heating process precluded the possibility of using plastic film substrates commonly used for photographic films and other plastics.
In patent application Ser. No. 055,270 E. W. Bouldin and J. Drexler described a reflective data storage medium derived from silver-halide emulsion and using a silver diffusion transfer process. No heating was required to create the reflective surface; reflectivities up to 24.4% to green light were achieved. However, the recording sensitivity of this material was less than that of the process described in application Ser. No. 012,235, which yielded reflectivities up to 17.2%.
In patent application Ser. No. 072,908 by J. Drexler and E. W. Bouldin, a medium was disclosed in which the recording sensitivity was greatly improved over that described in application Ser. No. 055,270 and even somewhat higher than that achieved in the medium described in application Ser. No. 012,235. However it was necessary to add an annealing step at a temperature at 250.degree. C. and above to achieve the desired results. Also, although the recording sensitivity was very good with a green laser beam at 514 nanometers and with a red laser beam at 633 nonometers, it fell off by about a factor of three when the laser wavelength was increased to 830 nanometers. This effect was similar to that observed with materials produced by the method of patent application Ser. No. 012,235. By the method of patent application Ser. No. 072,908 the best sensitivities were achieved with media having reflectivities in the green of 25.5% although reflectivities up to 36.6% were observed from less sensitive samples.
These last three referenced co-pending patent applications described a reflective data storage or laser recorcing media produced from silver-halide emulsions so as to create the desired reflective but non-electrically conducting surface desired for efficient laser recording. These photographic materials have the added advantage that photographic-type techniques can be used for either replicating master discs or pre-recording data or control markings on the reflective surface. However, these media were limited in recording sensitivity in the long wavelengths near 830 nanometers, and in achieving high reflectivity along with high sensitivity and in requiring a relatively high temperature process to be used for high sensitivity DRAW recording applications, which limited the selection of plastics that could be used as substrates.
An object of the invention was to devise a non-toxic, highly sensitive reflective DRAW laser recording and data storage medium which may be manufactured without the use of a vacuum system and on a continuous basis and which may be used to record low-reflectivity spots in a reflective field with relatively low energy laser pulses. Another object was to achieve at the long red wavelength and near-infrared wavelengths, recording sensitivities similar to what have been achieved by the recording media described in co-pending patent applications Ser. No. 012,235 and Ser. No. 072,908 in the visible. Another object was to devise a manufacturing process which would permit the use of a wider variety of plastic substrates. Another object was to achieve higher reflective surfaces without lowering the recording sensitivity so as to increase the reflective light to facilitate automatic focusing and increase the signal level when data is read.